U.S. and European authorities must combat a “global crisis” of Islamic extremists travelling to Syria and returning home as potential terrorists, U.S. Attorney General Eric Holder said.

“The United States and countries around the world face a new threat—the possibility that violent extremists fighting today in Syria, Iraq, or other locations may seek to commit acts of terror tomorrow in our countries as well,” Holder said in remarks prepared for delivery at the U.S. ambassador’s residence Tuesday in Oslo.

U.S. and European officials have reported a surge in citizens travelling to fight in Syria. Holder said the U.S. estimates that more than 7,000 foreign fighters are operating there. Among those are dozens of Americans and hundreds of Europeans, according to U.S. and European estimates. A 22-year-old Floridian blew himself up in a suicide truck bombing in Syria last month.

Holder was meeting today with Norwegian officials, including Prime Minister Erna Solberg. Later in the week, he travels to London for a meeting with officials from the U.K., Canada, Australia and New Zealand.

“We have a mutual and compelling interest in developing shared strategies for confronting the influx of U.S.- and European-born violent extremists into Syria,” he said. “Because our citizens can freely travel, visa free, from the U.S. to Norway and other European states—and vice versa—the problem of fighters in Syria returning to any of our countries is a problem for all of our countries.”

Norway last week proposed to tighten laws to hinder private citizens from taking part in armed conflicts abroad. Norwegian police in May arrested three citizens on terror charges, saying the men represent a threat to the country and its allies. They were apprehended to “prevent additional support of, or adherence to” the Islamic State in Iraq and the Levant. One of the men was released last month, while two remain in custody.

The U.S. Transportation Security Administration last week ordered tightened screening at some overseas airports with direct flights to the U.S. While the agency didn’t specify which ones were affected, London Heathrow, Europe’s busiest, stepped up security checks, including telling passengers it wouldn’t allow them to carry on electronic devices unless they showed that they worked.

Holder said combating the threat requires aggressive prosecutions, undercover probes, better sharing of traveler information between countries, and reaching out to Islamic communities.

“The Syrian conflict has turned that region into a cradle of violent extremism,” Holder said. “The world cannot simply sit back and let it become a training ground from which our nationals can return and launch attacks.”

FBI Director James Comey told reporters in May that the risk is “getting worse just because with time more people are traveling there from here and trying to travel there from all over the world. There are thousands and thousands of foreign fighters in Syria.”

Last month, Comey said he felt his European counterparts were “focused on this all day every day, trying to track those who are going and those who might come back.”